Traditional foam-in-place structural materials known in the art generally disclose polyurethane materials and epoxy-based materials. These materials incorporate a method to create volumetric expansion and a curing agent as well effectuate curing at room temperature and achieve a degree of control of expansion and cure rate characteristics. Although these prior art materials are both useful and successful in a number of applications, certain structural reinforcement applications in the automotive industry, for example, would benefit from a material having improved mechanical properties, such as a higher compressive strength, little change in modulus over a broad temperature range and a glass transition temperature that exceeds 200° F. In addition, improved cured ductility that would then enable the material to deform plastically would provide definite benefit. Further, these structural reinforcement applications in many applications, including automotive, may also benefit from a shear-thinning structural material which exhibits an increased viscosity at a zero shear rate and a decreased viscosity at a higher shear rates prior to curing. This enables the material to move as a fluid while being dispensed but then have minimal fluid following dispensing. This shear thinning behavior can also assist with the development of a uniform, consistent foamed cell structure by allowing more effective foaming gas entrapment.
As known by those skilled in the art, a number of factors determine the suitability of a process for forming a foamed product of the type in which a blowing agent forms cells in a synthetic resin as the resin is cured. Most significantly, the interaction of the rate of cure and the rate at which the blowing gas is generated must be matched to create the proper cured product. If the resin cures too rapidly there is inadequate time for the gas to form the proper size and number of gas voids in the finished product. Over expansion of the forming foam product must also be avoided. Rapid expansion due to a slow cure rate relative to gas evolution may cause the expanding foam to simply collapse as a result of inadequate wall strength surrounding the individual gas cells.
A number of prior art techniques are available to control the rate of foam expansion and the cure rate. For example, a wide-range of reactivities are available in commercial resins and curing agents. In addition, resins are available in a range of viscosities, which is another parameter, which can be used to control foam expansion rate. That is, it is known that a low viscosity resin can generally be expanded to a greater volume with a given volume of gas than a higher viscosity material; however, the resin must have sufficient viscosity to contain the gas at the pressures at which it is generated in order for the foam to be properly formed.
With respect to automotive applications, foamed products must have good environmental resistance and, most significantly, in many applications they must protect metal from corrosion while maintaining adhesion to the substrate. In the past many foamed parts were made using polyurethane, which provides a number of desirable attributes. It is known, however, that alternatives to urethane-based foams or more precisely materials based on the reactivate of the isocyanate chemical functional group are frequently more environmentally desirable, in part due to the potential for unreacted functional groups in the finished products and difficulty in handling isocyanate functional chemicals in manufacturing processes. In addition, the polyurethane materials found in the prior art fail to provide optimum mechanical properties, generally possessing lower elastic modulus strength and lower glass transition temperature than what is capable with epoxy-based materials. In comparison with polyurethane materials, however, the epoxy-based materials found in the prior art often exhibit both poor cured ductility and higher viscosity during dispensing.
Accordingly, there is a need in industry and manufacturing operations for a structural material, which exhibits improved mechanical properties, such as higher compressive strength, compressive modulus, glass transition temperature, better-cured ductility, combinations thereof or the like. The improved mechanical properties allow the structural material of the present invention to plastically deform without significant reduction in modulus or glass transition temperature as compared with typical epoxy-based materials. In addition, there is a need for an improved material, which can be used in a variety of applications wherein one or both components utilize a thixotropic filler, which produces pronounced shear-thinning characteristics. By providing a material with excellent cured physical properties and desirable processing attributes, the present invention addresses and overcomes the shortcomings found in the prior art.